A great number of optical systems require that the focal length of the optical system be variable so that the focus of the optical system can be changed. Often, the change in focal length is achieved by moving the optical elements toward or away from each other or the image plane. Linear movement of the optical elements or of the image plane is typically achieved by manual operation or through the use of motors.
Some optical elements require a very rapid change in focus, for example a focus change of one diopter in a fraction of a millisecond may be required. This rapid focus change is very difficult to achieve with linear motion due in part to the moment of inertia of the optical elements and/or the image plane. For example, typical mechanical systems of moving lenses may require hundreds of milliseconds to change the focus by one diopter.
One technique for changing focus is with the use of a pair of Alvarez plates, sometimes referred to as shear plates. A shear plate in this case is a transmissive plate having a flat, planar surface on one side thereof and a non-planar surface on an opposite side thereof that follows a polynomial function. In other words, the thickness of the plate varies with a polynomial function. These two shear plates are placed facing each other such that the surfaces following the polynomial function are opposed to each other. Furthermore, the facing surfaces are complementary to each other so that if the shear plates were to be brought into contact with each other, these surfaces would mate and opposite sides of the shear plates would be parallel to each other.
The shear plates are actually spaced apart from each other a sufficient distance such that they can be moved relative to each other in a direction that is orthogonal (transverse) to the optical axis (the primary axis along which light passes through the optical system). By moving the shear plates relative to each other, a change in focus can be achieved. Unfortunately, as with other approaches discussed above, the moment of inertia of the shear plates and mounting structures for moving the shear plates reduces the speed with which the shear plates can be moved relative to each other.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.